Unfortunately, that isn't really possible without a re-write. The way txt2lrf works is that it uses Markdown to convert plain text into HTML, then converts the HTML into an LRF. Not processing Markdown markup means no HTML, which means nothing to convert into an LRF.

You may want to check out pielrf. I'm not sure that it's still maintained, but the last time I looked at it it worked, and seems to do what you want -- directly convert plain text to LRF with minimal special markup interpretation.